Rapid microbial testing by ATP bioluminescence demands assays that will work in a variety of sample matrices. The microorganisms can be determined by lysing them and measuring the light emitted in the presence of luciferase, as a function of the ATP contained in the microorganisms. However, before detection can take place, it is often necessary to separate the organisms from the remainder of the sample, which may contain contaminating amounts of ATP.
Separation of the microorganisms from their environment poses particular problems in aqueous samples also containing fatty material, e.g. milk. It has been proposed to centrifuge such samples with a clearing agent in a centrifugation tube, to give a pellet comprising the intact microorganisms, supernatant liquid and, above that, a fatty layer. The fatty layer and the liquid can be removed by aspiration, but the removal of fatty material is inefficient and time-consuming. It is also unsuited to ready use by unskilled personnel, e.g. for simple testing of samples of milk at a large dairy or distribution depot. If the sample is cream, which may contain 40% fat or more, the problems are greatly magnified and the technique of aspiration is not considered suitable.
Centrifugation tubes are known, which additionally comprise, as a sliding fit therein, a less deep inner tube whose base is a filter material. Sample is introduced into the tubes when fitted together, and centrifugation enhances filtration, leaving solids on the filter.